FIG. 1 shows a configuration of a standard mobile packet communication system consisting of an exchange apparatus 1, a radio network control apparatus 2, base stations 3-1, 3-2, 3-3, . . . controlled by the radio network control apparatus 2, and mobile stations 4-1, 4-2, . . . wirelessly communicating with one or a plurality of the base stations and possessed by users, respectively.
TABLE 1Correspondence between transport channels andphysical channelsTransportPhysicalchannel namechannel nameOfficial nameDCHA-DPCHAssociated-DedicatedPhysical ChannelDCHDPCHDedicated PhysicalChannelDSCHPDSCHPhysical DownlinkShared Channel
This system achieves mobile packet communication by efficiently utilizing frequencies and by considering a characteristic of packet communication that a certain extent of delay is allowable. FIG. 2 shows a communication technique usually employed by the system. The base station 3 transmits data pieces D1, D2, and D3 to the user mobile stations 4-1, 4-2, and 4-3, respectively, by temporally multiplexing the data pieces on a single radio channel PDSCH (Physical Downlink Shared CHannel).
This specification uses as a radio channel name a physical channel name instead of a transport channel name. Table 1 shows correspondence between transport channel names and physical channel names.
To allow a single radio channel to be shared by a plurality of users, a prior art employs a channel configuration shown in FIG. 3. Separately from a shared channel PDSCH for transmitting data (downlink user data) for user mobile stations, there are A-DPCHs (Associated-Dedicated Physical CHannels) in an uplink and a downlink, respectively, to control transmission power. The uplink also involves a physical channel DPCH (Dedicated Physical CHannel) to multiplex uplink user data and control information sent from a mobile station.
According to the communication system of the prior art, the physical channel A-DPCH in the downlink only has a task of transmitting control signals, and therefore, is set at a relatively low channel speed. On the other hand, the physical channel DPCH in the uplink transmits control signals as well as user data, and therefore, is set at a channel speed higher than the downlink physical channel A-DPCH. The shared channel PDSCH in the downlink is set at a channel speed that is further higher than the downlink physical channel A-DPCH, to carryout high-speed data communication.
When conducting communication with the use of the high-speed shared channel PDSCH, a PDSCH transmission notice is sent to a mobile station through the A-DPCH before transmitting data with a radio frame on the PDSCH. Upon receiving the notice, the mobile station confirms the PDSCH transmission, and then, starts receiving the data on the PDSCH.
In this way, according to the conventional mobile packet communication system, a base station notifies a plurality of mobile stations of shared channel PDSCH transmission through the downlink physical channel A-DPCH, and then, transmits user data through the PDSCH. Namely, user data for a plurality of users are transmitted to a plurality of mobile stations by temporally multiplexing the data. In FIG. 3, the downlink involves two channels, i.e., the physical channel A-DPCH and shared channel PDSCH. The PDSCH is not always set but is set only when notification is made through the A-DPCH.
Conventionally, the mobile packet communication mentioned above uses a technique called diversity reception (hereinafter referred to as “DHO”) to improve the line quality of the physical channel A-DPCH and shared channel PDSCH in the downlink. FIG. 4 shows a channel configuration of mobile packet communication employing the DHO technique. The base stations 3-1 and 3-2 simultaneously transmit signals through the downlink physical channels A-DPCHs and receive the uplink physical channel DPCH from the mobile station 4-1. However, no DHO is applied to the shared downlink channel PDSCH for transmitting user data because user data of a plurality of users are temporally multiplexed, and therefore, it is difficult to control data transmission timing in DHO. Accordingly, only one base station, e.g., the base station 3-1 uses the PDSCH for data transmission.
A code division multiple access (hereinafter referred to as “CDMA”) method is a radio communication technique employing codes to establish a channel. For such a technique, transmission power control is important and essential, A W-CDMA system (Wideband Code Division Multiple Access system) adopted by IMT-2000 employs a technique called high-speed transmission power control. The high-speed transmission power control technique is applicable not only to the W-CDMA system but also to TDMA and FDMA Systems.
FIGS. 5 to 7 show an operation of transmission power control for CDMA.
In FIGS. 5 and 6, the mobile station 4-1:
(1) calculates a signal-power-to-interference-power ratio (hereinafter referred to as “SIR”) of the downlink physical channels A-DPCH always transmitted from the base stations 3-1 and 3-2;
(2) compares a result of the calculation with a predetermined target value;
(3) if the comparison indicates that the calculated SIR is lower than the target value, generates a transmission power control command to command the base stations 3-1 and 3-2 to increase the transmission power of the A-DPCH, and if the comparison indicates that the SIR is greater than the target value, generates a transmission power control command to command that the transmission power of radio signals for the mobile station must be decreased; and
(4) transmits the generated transmission power control command to the base stations 3-1 and 3-2 on the uplink physical channel A-DPCH.
In FIGS. 5 and 7, the base station 3-1:
(1) calculates a SIR of the uplink A-DPCH from the mobile station 4-1;
(2) compares the SIR of the A-DPCH with a predetermined target value, like the above-mentioned case of the mobile station;
(3) if the comparison indicates that the target value is lower than the SIR of the A-DPCH, generates a transmission power control command to command the mobile station 4-1 to increase the transmission power of uplink radio signals, and if the comparison indicates that the SIR of the A-DPCH is greater than the target value, generates a transmission power control command to command that the transmission power of uplink radio signals must be decreased; and
(4) transmits the generated transmission power control command to the mobile station 4-1 on the downlink physical channel A-DPCH.
In this way, radio signal transmission power between the base station 3-1 and the mobile station 4-1 is adjusted to always maintain optimum transmission power.
On the other hand, the transmission power of the shared downlink channel PDSCH is controlled according to a value obtained by multiplying a transmission power value of the physical channel A-DPCH in the same downlink by a predetermined value, i.e., an offset value. This is because the transmission power of the downlink A-DPCH is considered to be always optimally controlled according to a transmission power control command transmitted from a mobile station and because the A-DPCH and PDSCH are simultaneously transmitted. Accordingly, it is possible to consider the A-DPCH and PDSCH are in the same radio channel state. The offset value is notified from the radio network control apparatus 2 to each of the base stations 3-1, 3-2, and 3-3 in the mobile packet communication system of FIG. 9.
The conventional mobile packet communication system, however, will cause a problem during diversity reception (DHO). This problem will be clarified with reference to FIG.
Each of the base stations 3-1 and 3-2:
(1) calculates, at each of the base stations 3-1 and 3-2, a SIR of the A-DPCH when receiving the uplink physical channel A-DPCH from a mobile station;
(2) compares it with a target value;
(3) generates a transmission power control command to command a transmission power increase or decrease; and
(4) transmits, from each of the base stations, the transmission power control command to the same mobile station through the downlink physical channel A-DPCH of the base station.
Accordingly, the mobile station receives the two transmission power control commands.
On the other hand, the mobile station:
(1) receives the two A-DPCHs from the separate base stations, combines the two A-DPCHs according to a predetermined procedure, and calculates a SIR of the combined A-DPCHs;
(2) compares the SIR with a target value;
(3) generates, according to a comparison result, a transmission power control command to command a transmission power increase or decrease; and
(4) transmits the transmission power control command on the uplink physical channel A-DPCH.
This A-DPCH is transmitted from the mobile station 4-1 simultaneously to the base stations 3-1 and 3-2. This A-DPCH is a single radio signal although FIG. 8 shows two lines representative of the signal. The separate base stations 3-1 and 3-2 receive the same A-DPCH from the mobile station 4-1.
According to the DHO technique, a mobile station receives signals from a plurality of base stations and combines the signals into one. As a result, combined gains improve the quality of the downlink A-DPCHs. Similarly, a base station can receive a radio signal from a mobile station at a plurality of locations and combine the received signals into one to improve the quality of A-DPCH.
When considering the downlink A-DPCHs during DHO, it is understood that the base stations 3-1 and 3-2 simultaneously transmit radio signals and the mobile station 4-1 combines the signals together. Accordingly, the transmission power of the downlink A-DPCH of each of the base stations 3-1 and 3-2 can be small to realize the same SIR as that obtained when no DHO is carried out (i.e., a single base station transmits the downlink A-DPCH). More simply, transmitting a signal from two base stations instead of a single base station can halve the transmission power of each base station.
During DHO, the transmission power of the downlink physical channel A-DPCH of each of the base stations 3-1 and 3-2 is reduced. However, the transmission power of the downlink shared channel PDSCH is calculated by multiplying the downlink A-DPCH of the base station 3-1, which transmits the PDSCH, by an offset value without DHO. This results in deteriorating the reception quality of the PDSCH during DHO.
The present invention has been made to overcome these conventional problems. An object of the present invention is to provide a technique of always maintaining good reception quality on a PDSCH during diversity reception in mobile packet communication that transmits user data for a plurality of mobile stations by temporally multiplexing the user data on the single radio channel PDSCH.
Another object of the present invention is to provide a technique of always maintaining good reception quality on an HS-SCCH (High Speed Shared Control CHannel) of an HSDPA (High Speed Downlink Packet Access) method that fixes the transmission power of a data transmission channel and variably controls data quantities.